Power amplifiers

ABSTRACT

A method of controlling output power in a power amplifier having a driver stage and an output stage. The driver current is measured and the output stage biased in dependence upon the measured driver current.

The present invention relates to power amplifiers.

BACKGROUND OF THE INVENTION

In GSM like systems, the output power in a transmit mode of the transmitter is controlled by a power amplifier (PA). Typically, power amplifiers are controlled by controlling the current drawn by the power amplifier. This current is proportional to the output power. Alternatively, other parameters proportional to the output power can be regulated.

By sensing the current supplied to the power amplifier, and feeding it back to an error amplifier for feedback to a control input of the power amplifier, the biasing of the power amplifier can be changed in order to regulate the output of the power amplifier. Typically a filter would be used in the feedback loop in order to limit the amount of noise introduced into the system.

However, the typical transfer function of the power amplifier, or its gain (control signal to current consumption or output power) is not constant. When a mismatch occurs at the antenna, the load on the power amplifier is changed. This also means that the output power error gets bigger for a given voltage standing wave ratio (VSWR) level. As mentioned above, typically the DC current supplied to the power amplifier is regulated, but this means that the relationship between power out and DC current is only known if the load at the output at the PA is constant. As the load of the antenna varies, the output power from the PA varies differently for different phases at high VSWR. For example, at VSWR 8:1 for the different phases of the output power could vary by as much as +/−9 dB if there were not other limitations that limit the power output. This can be shown to be the case due to the expression P=I²R_(L)/2, where in real terms R_(L) varies from 6.25 ohms to 400 ohms at VSWR8:1. Accordingly, if better control is required, then information about the output power or the reflected power has to be used instead of the total DC power supplied to the power amplifier.

SUMMARY OF THE PRESENT INVENTION

It is therefore desirable to obtain improved control of the output power with a varying load. This is especially the case with very narrow band antennas that can vary in band from 1:1 to 5:1 in VSWR.

According to the present invention, there is provided a method of controlling output power from a power amplifier, the method including measuring DC current supplied to a driver stage of the power amplifier and providing a bias control signal for the driver stage and an output stage of the power amplifier in dependence upon the measurement of the DC driver current.

It is emphasised that the term “comprises” or “comprising” is used in this specification to specify the presence of stated features, integers, steps or components, but does not preclude the addition of one or more further features, integers, steps or components, or groups thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a power amplifier circuit embodying present invention; and

FIG. 2 illustrates current and output signals from the circuit of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a block diagram illustrating a power amplifier circuit embodying the present invention. The circuit includes an RF input 2 which is connected to an output stage power amplifier 10 via a driver stage 12 and an RF matching stage 16. An output of the PA output stage 10 is connected to the RF load 14 (usually an antenna). In an embodiment of the present invention, a DC current detector 20 serves to detect the DC current supplied to both the driver stage 12 and the output stage 10. The current detector 20 is supplied by a battery 21. The DC current detector 20 produces an output signal equivalent to I_(driver)+K*I_(output) for supply to a regulator 22. The choice of value for K is discussed below. The regulator 22 receives a reference current input 24 and it combines this with the output of the DC current detector 20 to produce a signal for supply to a bias control signal unit 26. The bias control signal unit 26 provides bias signals for the driver stage 12 and the output stage 10. These bias signals (28 and 30 respectively) serve to bias the input to the driver stage 12 and the output stage 10 respectively.

The output of the driver stage 12 is connected to the input of the output stage 10 by way of an RF matching circuit 16. The matching circuit is intended to match a low impedance of the output stage input to a higher load impedance at the output of the driver stage. The matching circuit should ideally be selected to have zero degrees phase shift, or such a phase shift that a high ohmic load at the output of the output stage transfers to a low ohmic load at the driver stage output.

Typically, the output transistor is an inverting component and a lower current swing (at higher ohmic loads) gives a lower in-phase feedback voltage to the input coming from the emitter inductance. A higher voltage swing at the output lowers the impedance through the capacitance between the base and emitter of the output stage. The feedback paths all work in the same direction so that a high ohmic load translates into a low ohmic load at the input and vice versa.

If the matching network between the input of the output transistor and the output of the driver stage is achieved with zero degrees phase shift, then a high ohmic load at the output transistor is translated to a low ohmic load at the driver stage. This means that for a high ohmic load at the output stage, the driver will have an increased DC current and the output stage will have a decreased DC current if the sum of them is kept constant. The output power can then be increased until either the voltage limit, the swing or until the current density in the driver limits the maximum current consumed, or until the total DC current is reached for small VSWR's. This situation can be very dramatic since at high VSWR the gain in the output stage can even be negative.

Accordingly, in embodiments of the present invention, the driver stage current is used to regulate (bias) the output stage and the driver stage, as described.

FIG. 2 illustrates that the driver current can be a better variable to control output power than the total current when the load varies. FIG. 2 a illustrates the driver stage and output stage currents, whilst FIG. 2 b illustrates the output power variation.

Ideally, both the driver stage and the output stage should be run in a saturated mode.

The DC current detector which outputs I_(driver)+K*I_(output) should be such that K is chosen so that the output power of the output stage becomes ideally a straight line. K would typically be chosen dependent on many factors such as gain of the output stage and the efficiency in the various different stages of the amplifier. 

1. A power amplifier comprising: a driver stage having an input for receiving RF input signal, and an output for supplying a driver signal; an output stage having an input for receiving a driver signal, and an output for supplying an RF output signal; and a bias control unit which is operable to measure electrical current supplied to the driver stage to produce a measured driver current value therefrom, and to supply a bias signal to the output stage in dependence upon the measured driver current value.
 2. An amplifier as claimed in claim 1, comprising an RF matching stage connected between the output of the driver stage and the input of the output stage.
 3. An amplifier as claimed in claim 1 or 2, wherein the bias control unit is operable to measure a current supplied to the output stage to produce a measured output current value therefrom, and to supply a bias signal to the output stage in dependence upon the measured driver current value and measured output current value.
 4. An amplifier as claimed in claim 1, 2 or 3, wherein the bias control unit is also operable to apply a bias signal to the driver stage in dependence upon the measured driver current value.
 5. An amplifier as claimed in claim 3, wherein the bias control unit is operable to supply a bias signal to the driver stage in dependence upon the measured driver current value and in dependence upon the measured output current value.
 6. An amplifier as claimed in claim 4, wherein the RF matching stage produces substantially 0° of phase shift between the output of the driver stage and the input of the output stage.
 7. An amplifier as claimed in claim 6, wherein the RF matching stage produces such a phase shift that a high ohmic load at the output of the output stage transfers to a low ohmic load at the driver stage output.
 8. A method of controlling the output power of a power amplifier having a driver stage for receiving a radio frequency input, and an output stage for applying a radio frequency output signal, the method comprising measuring a driver current and biasing the output stage in dependence upon the measured driver current. 